I'm posting this just as a stand alone item which may help anyone with oil pressure problems on a Volvo MD1B - I expect the same oil pump is used on the other MD series ( 1, 2, 3)

I had intermittent oil pressure on starting - occasionally the low pressure warning stayed on and the pressure stayed to near to zero - shut down quickly!

This happened pretty randomly but seemed more likely when starting after a period at higher revs - which may have been days or minutes ago.

I found that the over-pressure valve was sticking, very slightly, in the open position. This valve is inside the oil pump, accessible under the pump's top cover. It' looks like a round chrome vanadium plug held inside its channel with a spring. I found that for correct operation this plug must move smoothly in its channel with NO friction. I had to carefully smooth out the channel wall with fine wet & Dry then thoroughly flush out with paraffin. Then the plug slid smoothly in and out. Now the spring easily returns the plug to the 'closed' position without shunting oil past the pump.

It seems that greater oil pressure developed during higher revs was moving the plug higher up the channel, then it was sticking.